In order not to interrupt the narrative of the Yale experiments, we have foregone defining certain of the technical terms which it was necessary to use. It will be well, before going further, to give a simple description of the manner in which the food we eat is transformed in the body into tissue building material and energy: a process the many parts of which are grouped by physiologists under the name of Metabolism.
When you take a mouthful of food it enters on a journey through the body in which it traverses more than thirty feet of the intestinal tube before that part of it which the body cannot use is ejected; the process of metabolism begins the moment the lips touch it. The six salivary glands which are located in the mouth manufacture saliva, which flows out through numerous openings, and mixes with food as it is chewed. The saliva not only moistens the food, thus allowing it to be more easily swallowed, but it also has a most important chemical office, converting all starchy food matter into sugar, and thus performing the first and one of the most essential steps in the process of digestion.
After the food has been masticated and saturated with saliva, it passes down the throat through the gullet, which performs a peculiar muscular contraction, thrusting downward the particles of food. The conversion of the starch in food into sugar, or glucose, which is begun by the saliva in the mouth, is continued as the food passes into and down the gullet, but stops almost completely when the food once reaches the stomach.
THE WORK OF THE STOMACH
It is in the stomach, on the other hand, that most of the work of digesting the albuminoids, or proteids, of food is performed by the gastric juice. The stomach is a pear-shaped bag, that holds about three pints of material, or three-quarters of an ounce for every inch of the individual’s height. Food enters it through the gullet on the upper left hand side, just below the heart. Myriads of glands in the walls of the stomach are active in the formation of either pepsin, or an acid fluid which, when combined with pepsin, forms the gastric juice.
At the back of the stomach, partly overlapping it, lies the liver, which discharges a liquid called the bile into the alimentary canal just below the stomach. Behind the stomach, lies a large gland called the pancreas, which discharges a remarkable fluid, named pancreatic juice, into the intestine through the same opening which the liver uses for its bile. Connected with the stomach is the small intestine, which is the narrow portion of the alimentary canal, and the largest and most important of all the digestive organs. It is some twenty-five feet in length, and its walls are everywhere covered with glands which secrete and exude mucous and other fluids.
At the lower end of the intestine is the colon or large intestine which is not a digestive organ in itself, but is a reservoir in which the food is stored up for a short time, to allow opportunity for complete absorption of the digested portions.
THE ELEMENTS OF FOOD
Although there may be thousands of different dishes, and combinations of foodstuffs, fundamentally they are all closely akin, and can be all resolved into a few quite simple elements: Proteid, Carbohydrate, Fat or Mineral Salts, or combinations of these; the Proteid class having many subdivisions, and the Carbohydrates being made up of the various sugars and starches.
We also know definitely just what use the body makes of these various substances. The Proteid is the up-builder of tissue, the essential foodstuff without which life cannot exist. If we compare the human body to an engine, as nearly all physiologists seem bound to do, we may say that Proteid is like the brass, or other metal, of which the structure is composed. The various Carbohydrates and the Fats are the fuels from which are derived the energy which animates and operates the mechanism. The Mineral Salts are used to supply various important bodily needs, such as elements required by the bones, or the delicate tissue in the eyes, the enamel for the teeth, and so forth.